In a typical fine blanking apparatus, it is desired to accurately cut and punch a part. Such apparatus usually comprises an upper die and a lower die with one or more punches associated with the upper die and a punch base associated with the lower die, and movable with respect to the lower die. In the forming of parts in order to insure the desired precision, it has been common to provide a delay in the movement of the punch base upwardly after the part has been cut and punched from the workpiece. Such a time delay has been produced by cam control of the punch bases or by a hydraulic system associated with the punch bases. A typical example of a hydraulic system is shown in U.S. Pat. No. 3,570,343.
Such hydraulic systems have a disadvantage in that they require associated hydraulic fluid lines, valves and the like externally of the hydraulic cushion on the press.
Among the objectives of the present invention are to provide a die stamping system which can be provided within the confines of conventional apparatus and is especially applicable for fine stamping; which will provide the desired time delay; wherein the time delay can be readily adjusted as desired without disassembling the system and wherein the delay action is positive.
In accordance with the invention, each cylinder assembly includes a first piston associated with and exposed to the inert gas such as nitrogen in the manifold and a second piston engaged by the first piston and urged outwardly into engagement with a punch base. A hydraulic circuit is associated with the second piston and controlled by a valve such that upon downward movement of the first piston hydraulic fluid may flow freely without inhibiting the movement of the first piston, but upon actuation of the valve hydraulic fluid locks the first piston and thereby prevents it from moving upwardly until the valve is actuated so that a predetermined time delay is provided.